Ultrasonic devices are used to diagnose and treat cardiovascular disorders. Ultrasonic imaging can be performed in the non-invasive manner of transmitting acoustic waves into a body of interest and detecting the resulting reflections of the waves. The reflections occur as a result of changes in acoustic impedance, so that time delays in receiving the reflections or phase changes of the reflections can be used to form images of organs or other tissue within the body of interest. However, non-invasive ultrasonic imaging has a number of limitations. For example, adequate resolution of target tissue deep within the body is limited by accessibility of the tissue to the acoustic beam and by the frequency of the beam. The target tissue may be inaccessible through the bone and/or lung structure and closer tissue of a patient. With regard to frequency, higher frequencies provide the short wavelengths that enhance resolution, but the higher frequencies attenuate more rapidly, limiting the depth of penetration.
In some applications, intracorporeal ultrasonic devices overcome these limitations. A single transducer element or an array of transducer elements within a probe can be used to transmit and receive acoustic beams when placed within the body of interest. Catheter probes provide a 360.degree. cross-sectional image of a vessel by passing the probe through the vessel. Both mechanical and electrically switched catheters are known. U.S. Pat. No. 5,273,045 to Chihara et al. describes both types of catheters. In one embodiment of a mechanical catheter, a single piezoelectric element is directed at a 90.degree. angle to the longitudinal axis of the catheter, and the element is caused to rotate to provide a full-circle image. In a second embodiment of a mechanical catheter, a single transducer element directs an acoustic beam along the longitudinal axis of the catheter, and a reflector is used to deflect the beam at a 90.degree. angle to the axis. The reflector is mechanically rotated to image the entire surrounding area. Turning to electrically switched catheters, the Chihara et al. patent describes and illustrates a conventional electrically switched catheter as having an array of piezoelectric elements arranged about the circumference of a cylindrical region of a probe tip. Excitation of the piezoelectric elements is phased and sequenced to image the region surrounding the probe tip. While each of the embodiments provides transverse scanning, look-forward and look-rearward catheters can be formed with only relatively minor adaptations. Chihara et al. also describe an embodiment in which imaging is directly forward of the path of the catheter. Other intravascular devices that utilize multiple transducer elements are described in U.S. Pat. Nos. 5,284,148 to Dias et al. and 5,291,090 to Dias, which are assigned to the assignee of the present application.
For the conventional electrically switched catheter, the transducer elements along the outer surface of a cylindrical region of the catheter are selectively triggered and phase shifts are introduced in order to generate a generally planar wavefront. However, only a limited number of transducer elements can be triggered at a single time. As an example, if a 32-element catheter is employed, the 16 elements that are on the side of the catheter opposite to the direction of the wavefront must remain inactive. Of the other 16 elements, use of the elements that are at the greatest angle to the direction of the wavefront requires large phase shifts, with image-generating contributions of these elements being highly apodized because of shading effects. Thus, in practice the maximum number of elements that can be activated at a given time are only those elements within a 90.degree. sector. If the imaging rotation is in a clockwise direction, progression takes place by electrically exciting the previously unexcited element beyond the clockwise end of the 90.degree. sector and terminating excitation of the element at the counterclockwise end of the original 90.degree. sector. That is, the 90.degree. sector progressively sweeps about the transducer array by initiating excitation of a front-end element and terminating the excitation of a back-end element.
While the prior art method of forming and operating an electrically switched catheter functions well in many applications, the arrangement has some limitations. As previously noted, a relatively small fraction of the total number of transducer elements is used at one time. Moreover, the beam-forming characteristics of the circular array are significantly less than optimal. It is difficult to image structures that are in near contact to the imaging ultrasonic array. The close proximity of the array to the structure of interest increases the adverse effects of the imaging component sometimes referred to as the "ring-down artifact." This phenomenon results in poor resolution in the near field of the scan.
What is needed is an ultrasonic probe, such as a catheter, which provides high resolution full-circle imaging even in the near field.